One of the main goals of the Beam Energy Scan (BES) Program at the Relativistic Heavy Ion Collider (RHIC) is to explore the phase diagram of the Quantum Chromodynamics (QCD) [1]}. At vanishing and small values of the chemical potentials for conserved charges, such as net-baryon, net-charge and net-strangeness, Lattice QCD has predicted that it occurs a smooth crossover from the hadronic phase to the QGP phase [2]}. Based on Lattice QCD calculations with physical values of light and strange quark masses, the negative signal of ratio of the sixth order to the second order (\(C_6/C_2\) ) is observed in the crossover region [3]}, [4]}, [5]}, [6]}. The QCD-assisted low-energy effective theory and the QCD based models, such as the Polyakov loop extended quark-meson (PQM) and the Nambu-Jona-Lasinio (PNJL) models, also support that the sixth order cumulant is negative near the chiral crossover transition [7]}, [8]}, [9]}, [10]}, [11]}. Consequently, \(C_6/C_2\)  is a powerful observable to study the QCD phase diagram in experiment.
